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published online 6 September 2012Journal of Fire Sciences

Morgan and Pedro H H AraújoTaís Felix, Orlando P Pinto, Jr, Augusto Peres, Joao M Costa, Claudia Sayer, Alexander B

Decabromodiphenyl Ether in Flame Retardancy of High Impact Polystyrene (HIPS)Comparison of Bismuth Trioxide and Antimony Trioxide as Synergists with

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Article

Journal of Fire Sciences

0(0) 1–9

The Author(s) 2012

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DOI: 10.1177/0734904112456004

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Comparison of bismuth

trioxide and antimony trioxideas synergists withdecabromodiphenyl ether inflame retardancy of high-impact polystyrene

Taıs Felix1, Orlando P Pinto Jr 1, Augusto Peres2,

Joa ˜ o M Costa2, Claudia Sayer 1, Alexander B Morgan3 and

Pedro HH Arau jo1

Date received: 5 May 2012; accepted: 5 July 2012

Abstract

Flame-retardant additives used in plastics are based on halogen, phosphorus, inorganic com-pounds, and minerals. In this study, the use of bismuth trioxide as synergistic agent with bromi-

nated flame retardant is investigated and compared to formulations with antimony trioxide.Decabromodiphenyl ether and metal trioxide were incorporated in a polymer matrix via single-screw extrusion. The samples were classified with the UL 94 vertical burning protocol and stud-

ied with thermogravimetric analysis. The general combustion and heat release behaviors werestudied via cone calorimeter. Results showed that 4% (wt) Bi2O3 displayed a synergistic effectwith 12% (wt) of the halogen compound to achieve V-0.

KeywordsFlame retardant, synergist agent, polymer

1Departamento de Engenharia Quımica e Engenharia de Alimentos–EQA/CTC, Universidade Federal de Santa Catarina,

Florianopolis, Santa Catarina, Brazil2Centro de Pesquisas Leopoldo Americo Miguez de Mello/PETROBRAS, Rio de Janeiro, Rio de Janeiro, Brazil3University of Dayton Research Institute, Advanced Polymers Group, Multiscale Composites and Polymers Division

Dayton, OH, USA

Corresponding author:

Taıs Felix, Departamento de Engenharia Quımica e Engenharia de Alimentos–EQA/CTC, Universidade Federal de SantaCatarina, Florianopolis, Santa Catarina 88040-970, Brazil.

Email: [email protected]








Introduction

Most modern plastics are made from hydrocarbon feedstocks and therefore are combustible.

When these materials are used in applications for buildings and structures, aircraft or public

transport, and electronic devices, these materials must be processed and formulated so as to

become difficult to ignite or burn near a heat source.1 High-impact polystyrene (HIPS) iscomposed of polybutadiene with grafting of polystyrene chains and is widely used for injec-

tion molding materials. For applications in electrical and electronic goods, additives are

incorporated into the plastic to give flammability resistance, and these are usually bromi-

nated compounds and a synergist due to their high cost-effectiveness and the fact that typical

loading levels of these additives do not negatively affect mechanical performance. Among

these brominated compounds commonly used for polystyrene, hexabromocyclododecane

(HBCD), and decabromodiphenyl ether (DECA) are the most commonly cited, even only

DECA is used in HIPS as HBCD is not effective in flammability tests for electrical and elec-

tronic goods. DECA has been the most widely used flame retardant for HIPS due to its low

cost and high loading of bromine (83%) in its structure. This high loading of bromine meansthat it is particularly effective at low loadings, which does not negatively affect other polymer

properties. This fact has led to DECA being optimized to work well with HIPS for flame

retardancy while not negatively affecting too many other polymer properties.

These brominated retardants are generally used in combination with synergists, especially

antimony trioxide, which in itself is not a flame retardant. As shown previously,2,3 the syner-

gists enhance the efficiency of the compounds, since the metal halide formed during the

combustion reacts on several occasions with the radicals formed by the burning of the poly-

mer. The particles of oxides or hydroxides resulting from these reactions can also catalyze a

recombination of the radicals, leading to an eventual termination of radical propagation,

which in turn causes the flame to self-extinguish. Group V of the periodic table (N, P, As,

Sb, and Bi) can carry out these types of reactions. Due to practicality of the chemistry of

these oxides to use in the polymers, N, P, and As are not used in oxide form but have shown

synergism with halogen. So Sb and Bi oxides are the only ones to be considered in this study

since their oxides are stable to melt compounding conditions and do not have strong toxicity

(such as arsenic oxide). Thus, SbX and BiX have a dual activity in reactions, such as flame

retarding and providing HX species and their metal oxides, since both promote a catalytic

removal of reactive radicals in the flame. This phenomenon has been well studied recently,4

and studies on this synergism go back even further to 1972–1986.5–9

In recent years, the pair of DECA and antimony trioxide has been widely used, especially

for polystyrene. Due to increasing demand of flame-retardant polymers, there is a need to

propose new reagents that can replace with similar efficiency, especially antimony trioxide,

obtained from mineral extraction. Bismuth oxide is found naturally as the mineral bismite

(monoclinic) in the form of crystals with colors from yellow green to yellow, mainly in

Bolivia, Mexico, and Japan. Among metals considered ‘‘heavy,’’ bismuth oxide has lower

toxicity and thus might be a good substitute for antimony trioxide as antimony trioxide gets

deselected due to environmental (European Electronic Waste Regulations) or cost issues.

In this work, the use of bismuth trioxide as synergist agent with a brominated flame retar-

dant (DECA) is investigated and compared to formulations with antimony trioxide in com-

positions with HIPS. The resulting composites are characterized with respect to thermal

stability (thermogravimetric analysis (TGA)) and flammability performance (Underwriter’s

Laboratory Test 94 (UL-94) vertical burning test and cone calorimetry).

2 Journal of Fire Sciences 0(0)








Experiment

The high impact polystyrene (HIPS) was produced by Innova S/A. Decabromodiphenyl

ether (DECA), with the trade name Saytex 102E of ALBEMARLE CORP., was used as the

halogenated flame retardant. Antimony trioxide (Sb2O3-abbreviated hereafter as SbO) was

provided by FORMIQUI ´ MICA and bismuth trioxide (Bi2O3- abbreviated hereafter as BO)

were purchased from FLUKA, and both evaluated as synergists. The polymeric material

(HIPS) was dried in an oven at 60C for 24 h to remove moisture from the material and

avoid interference during the extrusion process. The other reagents were used without drying

or any pretreatment.

The compositions (Table 1) were weighed and mixed manually. A single-screw extruder

of 14 mm L/D 30 from AX Plastics was employed in all extrusions. The temperature zonesranging from 190C to 197C with constant rotation of 45 r/min. Each formulation was

extruded three times to ensure good incorporation of flame-retardant compounds in the

polymer matrix.

The flammability tests were performed following ASTM D3801-06,10 by burning in a ver-

tical position. According to the standard, we tested five specimens with defined dimensions

(125 mm 3 13 mm 3 5 mm) for each formulation. This standard is used for the UL-94,

which used to rate the fire safety performance of plastics used in electronic and electrical

goods. It should be noted though that while we tested according to this protocol and used

the ratings listed in the test protocol, our results are not certified V-0, V-1, or V-2 ratings.

Rather, they are an approximation of what we believe the ratings would be if one followsASTM D3801-06 and submitted the samples to a UL-94 certified testing lab. TGAs were

conducted on a Shimadzu TGA-50 equipment, under inert atmosphere with a N2 flux of 50

mL/min and heating rate of 10C/min, until the final temperature, 600C, was reached.

Combustion behavior was assessed according to ASTM E 1354-10 using a Fire

Testing Technology (FTT) dual cone calorimeter apparatus. The samples were irradiated at

35 kW/m2, and the exhaust gas flow rate was 24 L/s. Samples were molded on a heat press

with defined dimensions (100 mm 3 100 mm 3 5 mm) and were wrapped in aluminum foil

and tested without frame and grid. The only deviation from the ASTM method was to test

each sample once rather than in triplicate as per the standard. This was done due to the lim-

ited amount of material available. Since we believe the samples to be homogeneous in com-position, we believe that the single-point data are still within the standard 10% error for the

Table 1. Formulations of the samples

Sample HIPS (wt%) DECA (wt%) Sb2O3 (wt%) Bi2O3 (wt%) Sb2O3 (mol%) Bi2O3 (mol%)

HIPS 100 — — — — —

SbO 6:2 92 6 2 — 0.68 — SbO 9:3 88 9 3 — 1.03 — SbO 12:4 84 12 4 — 1.37 — BO 9:3 88 9 — 3 — 0.64BO 9:4.8 88 9 — 4.8 — 1.03BO 12:4 84 12 — 4 — 0.86BO 15:5 80 15 — 5 — 1.07

HIPS: high-impact polystyrene; DECA: decabromodiphenyl ether.

Felix et al. 3








technique, but we admit that the error is ultimately unknown since we do not have triplicate

test data. However, the samples were well behaved during testing and should yield heat

release (HR) data within the 10% error.

Results and discussion

The derivative thermogravimetric analysis (DTG) curves for pure HIPS and the samples

SbO 12:4 and BO 12:4 with the same concentrations by weight of synergist are presented in

Figure 1. The decomposition of the pure polymer matrix is between 380C and 500C, with

a prominent peak at ;450C. For samples containing oxides, the decomposition follows dif-

ferent paths, occurring in two distinct regions. For sample SbO 12:4, containing antimony

oxide, the first decomposition peak appears at ;360C, starting at ;90C below the tem-

perature displayed by the pure matrix. This behavior indicates that the oxide begins to

degrade via reactions with the brominated flame retardant.11 The behavior of bismuth oxide,

in turn, can be considered similar to the curve of SbO 12:4, the difference being that the first

decomposition peak appeared at ;340C, about 20C below the temperature observed for

Sb2O3 in the same conditions. The lower temperature of degradation of the metal oxides,

such as Sb2O3 and Bi2O3, is important because it forms a volatile product that slows reac-

tions in the flame, and even though the oxide itself has no effect, it reacts with the halogen

flame retardant increasing its efficiency. The second peak appears at around ;415C–425C

and represents the degradation of the brominated flame retardant and the polymer.

Table 2 presents the results of flammability in the UL-94 vertical burning test. For the

sample of HIPS without any flame retardant, the specimens were not classified; they burned

completely, presenting dripping of the melting polymer on fire during burning. The sample

Figure 1. DTG analysis for samples of pure HIPS, SbO 12:4, and BO 12:4, in an inert atmosphere.DTG: derivative thermogravimetric analysis; HIPS: high-impact polystyrene.









SbO 6:2 also presented dripping, and the fire self-extinguished just after 30 s of being non-

classified by vertical UL-94 test. Sample SbO 9:3 burned after exposure to the flame but

self-extinguished in 2 s. Since the sample SbO 12:4 showed no afterglow in the char once the

flame extinguished, it was classified as V-0.

The sample BO 9:3 presented a similar behavior to SbO 6:2. It did not reach the UL-94

classification although their average burning time was 33 s. The samples BO 12:4 burned for

just 2 s, while samples BO 15:5 did not present the formation of flaming drip away from the

rest of the specimen, thus extinguishing the fire. However, it was observed that only the sam-

ples with burning time over 30 s (nonclassified) presented dripping and burned the cotton.

The phenomenon of incandescence (char afterglow) was not observed in any of the BO sam-

ples. When analyzing the results of UL-94 for the mixtures with equal weight concentrations

of flame retardants (samples SbO 9:3 and BO 9:3), it may be noted that the bismuth trioxide

was not as effective as antimony trioxide. Two possible explanations could be formulated to

explain this difference. The molecular weight of bismuth trioxide (465.96 g/mol) is consider-

ably higher when compared to antimony trioxide (291.52 g/mol). This means that the same

weight-based formulation has a molar concentration of bismuth trioxide much lower than

that of antimony trioxide. The second factor lays on the reactions in the vapor phase. The

reactions of bismuth trioxide and DECA in the condensed phase lead to less-volatile species

in relation to antimony halides, thus reducing the flame-inhibiting reactions in the gas phase

and decreasing the effectiveness of the material as a synergist agent. To verify the effect of

the number of moles of Bi2O3 at the composition, a new extrusion, BO 9:4.8, was performed

keeping the number of moles of Bi2O3 equal to the number of moles of Sb2O3 in the compo-

sition of SbO 9:3. However, BO 9:4.8 did not reach the UL-94 classification, indicating that

vapor phase reactions still play a major role in the action of synergism between DECA and

antimony trioxide or bismuth trioxide, and for whatever reason, BO does not produce the

same concentration of metal halide species in the flame essential to self-extinguishing beha-

vior. However, the vapor pressure of BiBr3 (should that be the species formed in these condi-

tions) is much lower than that of SbBr3, so if less of the metal halide is volatilized in the

small flame of the UL-94 ignition source, then perhaps this result is not that surprising. The

cone calorimeter data for all five samples tested are shown in Table 3. HIPS without any

flame retardant showed the higher flammability overall (Figure 2). The addition of Bi2

O3

and DECA yielded some decrease in peak heat release rate (HRR) and total HR but not as

great as that observed with samples with antimony trioxide. About vapor pressures and

Table 2. Results obtained for the test of flammability UL-94

Sample UL-94

HIPS NC

SbO 6:2 NCSbO 9:3 V-0SbO 12:4 V-0BO 9:3 NCBO 9:4.8 NCBO 12:4 V-0BO 15:5 V-0

HIPS: high-impact polystyrene; NC: not classified by UL-94 standard.

Felix et al. 5








molecular weights of the Bi and Sb halides, perhaps at higher heat flux, Bi may show more

of a flame-retardant effect (HR reduction) when it is successfully volatilized.

From the data in Table 3, we can see that the addition of DECA and SbO or BO results

in reductions in peak HRR and average HRR, but the results are mixed with regard to per-

formance. While the use of BO lowers peak HRR, it does not reduce total HR or average

HRR as much as the use of SbO does. Going back to the commentary on BiBr3 not being as

volatile as SbBr3 under fire conditions, we can see this in the flammability measurements, espe-

cially when studying the smoke release and effective heat of combustion measurements. The

use of SbO results in more vapor phase flame retardancy, which causes an increase in smoke, a

lower total HR, and a lower effective heat of combustion. Because SbO can form more volatile

antimony bromide, it is a more effective flame retardant. However, the drawback to this will be

an increase in smoke. BO on the other hand is not as effective as SbO in reducing HR, but it

generates less smoke than base HIPS, which suggests that provided BO meets other flammabil-

ity tests (such as UL-94 V), the use of BO may present a useful compromise performance of

vapor phase activity with a lower amount of smoke release during burning. More studies would

be needed though to verify this level of performance that improved fire safety could be obtained

with BO with lower smoke and a good balance of properties (mechanical, fire) in the final

-200

0

200

400

600

800

1000

1200

0 100 200 300 400 500

H R R ( k W / m ² )

me (s)

HIPS

BO 9:4.8

SbO 6:2

SbO 7.5:2.5

SbO 9:3

Figure 2. HRR analysis for samples of pure HIPS, SbO 6:2, SbO 7.5:2.5, SbO 9:3, and BO 9:4.8.HRR: heat release rate; HIPS: high-impact polystyrene.

Table3. Cone calorimeter data for polymer samples

Sample Time toignition (s)

Peak HRR(kW/m2)

Time topeak HRR (s)

AverageHRR(kW/m2)

Total heatrelease(MJ/m2)

Total smokerelease(m2/m2)

Average effectiveheat of combustion (MJ/kg)

HIPS 80 1085 208 638 153.8 6471 29.87SbO 9:3 74 886 243 381 72.1 9289 13.59SbO 6:2 71 740 234 403 96.7 8504 18.05SbO 7.5:2.5 76 893 240 394 79.1 8660 15.26BO 9:4.8 59 806 219 512 116.2 4762 20.60

HIPS: high-impact polystyrene; HRR: heat release rate.









product. Observed fire behavior during burning was as follows: Upon exposure to the heater,

the sample with HIPS slowly began to darken and give a surface appearance of being molten.

Later the sample began to smoke and then finally ignited. The sample burned with a high-

intensity sooty flame. As the sample began to burn itself out, the edges of the aluminum foil

curled up such that it was difficult to close the cone calorimeter heat shutters at the end of the

experiment. Final char (Figure 3) was mostly black soot and some char residue on the alumi-

num foil. All of the samples with HIPS and antimony trioxide showed the same fire behavior.

Upon exposure to the cone heater, the surface went from glossy to blistered (small bubbles

starting to form). The sample then began to smoke and flashed a few times before sustainedignition occurred. After ignition, the flames were unsteady and pulsating at the surface, and

underneath the flames, the sample could be seen to be boiling away, which is likely the cause

for the observed unsteady flames. After burning for a little while, the samples extinguished and

left behind some black/grey chars (see Figure 3). For sample SbO 9:3, the char was again

mostly at the edges of the aluminum foil holder, but the foil was intact and some network/web-

bing type chars could be made out (Figure 3(b)).

Fire behavior of the sample with bismuth trioxide was similar to that of the virgin HIPS,

with the sample yielding a very large vigorous flame during burning with some small flame-

lets coming out around the edge of the sample, but eventually the entire flame smoothed out

into a uniform single flame around 150 s into the test. Also, while the surface melted a bit, itdid show some surface roughness. Finally, right before extinguishment, a green flame was

observed. The final char showed a gold-colored char at the edges of the sample with black

char in the middle (Figure 4(a)). However, this char smoldered a bit after removal from the

cone calorimeter and once that char had fully cooled, almost all of the black ash had burned

away leaving the gold-colored residue (Figure 4(b)). The colors in the flames are likely from

some sort of bismuth halide species.

Conclusion

The results showed that the formulations with 88% HIPS, 9% DECA, and 3% Sb2O3

achieved a V-0 rating. However, the formulation with the same concentration of flame

Figure 3. Chars for samples (a) SbO 6:2 and (b) SbO 9:3.

Felix et al. 7








retardant but with bismuth trioxide instead of antimony trioxide was not classified by UL-

94 standards. The reactions of bismuth trioxide and DECA in the condensed phase lead to

less-volatile species in relation to antimony halides, reducing the reactions promoted in the

gas phase and decreasing the effectiveness of the material as a synergist agent. Increasing the

amount of flame retardants, 84% HIPS, 12% DECA, and 4% Bi2O3, the polymeric material

reached V-0 rating. The burning behavior was very similar to the specimens with 88% HIPS,

9% DECA, and 3% Sb2O3, indicating that the synergistic action of these agents are similar,

although Bi2O3 should be used in higher concentrations to get similar flame retardancy.

The addition of Bi2O3 and DECA yielded some decrease in peak HRR and total HRwhen compared to pure HIPS, but it was not as great as that observed with samples with

antimony trioxide. So while Bi2O3 does have some effectiveness as a brominated flame-

retardant synergist and should be less toxic than Sb2O3, it is not as effective as antimony

oxide and is not a 1:1 replacement for that synergist. Further, it shows some different burn-

ing behavior that may (lower smoke) or may not be useful (char smoldering in the cone

calorimeter) from a fire safety perspective. Still, if antimony oxide were to be deselected

from use due to economic or environmental reasons, bismuth oxide may have some viability

as a replacement provided it is kept in mind that it is not as effective per unit of weight.

Funding

This work was supported by PETROBRA ´ S/Centro de Pesquisas Leopoldo Ame ´ rico Miguez de Mello

(CENPES), Conselho Nacional de Desenvolvimento Cientı ´fico e Tecnolo ´ gico (CNPq), and

Coordenacxa ˜o de Aperfeicxoamento de Pessoal de Nı ´vel Superior (CAPES).

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polymers. Oxford: Clarendon Press, 1982, pp. 212.

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Figure 4. Final char for sample BO 9:4.8 (a) immediately after testing. The ‘‘copper-colored’’ spots are

actually smoldering char. (b) The final character after cooling.









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8. Costa L, Camino G and Trossarelli L. Thermaldegradation of fire retardant chloroparaffin-metalcompound mixtures: part III—a comparison of thebehaviours of antimony trioxide and bismuth compounds.Polym Degrad Stabil 1983; 5: 367–371.

9. Costa L, Camino G and Luda di Cortemiglia MP.Mechanism of condensed phase action in fire retardantbismuth compound-chloroparaffin-polypropylenemixtures: part II—the thermal degradation behavior.Polym Degrad Stabil 1986; 14: 165–177.

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Author biographies

Taı ´s Felix graduated in chemistry in 2007 from the Federal University of Santa Catarina (UFSC) in

Physical Chemistry of Surface Area. She received her masters in chemical engineering in 2010 at the

same university (UFSC) in Flame Retardants on Polymer Nanocomposites Area.

Orlando P Pinto Jr is a graduate student in chemical engineering at Federal University of Santa

Catarina (UFSC).

Augusto Peres is a Research Engineer at the Tecnologia Petroquı ´mica–Research Center Leopoldo

Ame ´ rico Migues de Mello (CENPES) /PETROBRAS.

Joa ˜ o M Costa is a Research Engineer at the Tecnologia Petroquı´mica Centro de Pesquisas Leopoldo

Ame ´ rico Miguez de Mello (CENPES)/PETROBRAS.

Claudia Sayer graduated in chemical engineering from the Federal University of Rio de Janeiro

(UFRJ) in 1992 and received PhD in Chemical Engineering from COPPE/UFRJ in 1999. She is

Adjunct Professor at Federal University of Santa Catarina (UFSC) since 2004.

Alexander B Morgan Received B.S. degree in chemistry from the Virginia Military Institute (1994) and

received a Ph.D. in chemistry from the University of South Carolina in 1998. He is distinguished

Research Scientist and Group Leader of the Advanced Polymers Group, Multiscale Composites and

Polymers Division at the University of Dayton Research Institute (UDRI)

Pedro HH Arau jo graduated in Chemical Engineering from Federal University of Rio de Janeiro

(UFRJ) in 1992 and received Ph.D. in Chemical Engineering from COPPE/UFRJ in 1999. He is

Adjunct Professor at Federal University of Santa Catarina (UFSC) since 2004.

Felix et al. 9

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